Hard disk drive systems include a rotating magnetic disk, and a read/write head which is supported adjacent the disk for movement approximately radially of the disk. The read/write head includes a portion which is a write head and which magnetically records data on the disk, and a portion which is a read head and which reads information from the disk. The analog output signal from the read head is supplied through a preamplifier to a read channel circuit, which processes the output signal from the read head.
One technique for increasing the density of information stored on a given hard disk is to use a magneto-resistive (MR) read head. The MR read head is used in conjunction with a write head which is linear, and which thus stores data on the disk without introducing nonlinear distortion. In contrast, the MR read head has a nonlinear transfer function, which is approximated by the cos.sup.2 function, and which can introduce distortion into the signal it reads from the disk. The amount of distortion depends in part on the degree of nonlinearity, which varies from head to head. In addition, the nonlinearity of the signal can be affected by system parameters, such as head flying height, disk material, disk rotational speed, head shape, and ambient temperature. The added distortion may cause errors when supplied to existing read channel circuits, and such errors can significantly degrade the read channel performance. Although it is known to use an automatic gain control circuit to normalize variations in a linear portion of the transfer function, nonlinear portions of the transfer function introduce distortion which cannot be corrected by an automatic gain control circuit.
One known approach for dealing with this problem is to purchase a greater quantity of MR read heads than is needed for production, and to then test each read head and discard those with the greatest degree of nonlinearity. Thus, the resulting subset of read heads, which are eventually installed in the actual products, have a limited degree of distortion and thus produce a reduced number of read channel errors. However, although this approach reduces errors, it does not always eliminate them. Moreover, discarding a large percentage of the purchased read heads is inefficient and expensive, and thus increases the cost of the resulting hard disk drive systems.
Another known approach is to provide a nonlinear analog-to-digital converter in the read channel circuit. The analog-to-digital converter is specifically designed with a nonlinear characteristic which is the inverse of the MR read head nonlinearity, and which compensates for the distortion introduced by the MR read head. However, analog-to-digital converters are complex and take up a good deal of area in an integrated circuit, thus increasing the overall cost and power consumption of the integrated circuit. Further, the distorted signal cannot be applied to the analog-to-digital converter until after it has been processed by an automatic gain control circuit and a low pass filter, thus degrading their performance. For similar reasons, a nonlinear analog-to-digital converter cannot be incorporated into the preamplifier which precedes the read channel circuit. Moreover, although a nonlinear analog-to-digital converter can be used in a read channel which relies on digital detection, it cannot be used in a read channel which relies on analog detection.